Engines are usually designed with the ability to deliver a peak output, although most engine operation is performed well below this peak value. As such, it can be beneficial to operate with some cylinders inducting air without fuel injection, and other combusting air and injected fuel, as described in U.S. Pat. No. 6,568,177. By disabling some of the cylinders on the engine when desired output, such as torque, is relatively low, the engine can be operated at a higher manifold pressure to supply the needed airflow to the operating cylinders, thus reducing engine pumping losses and making the engine more efficient. In one example, the cylinder can switch the sets of cylinders deactivated to provide even wear, etc. See, for example, U.S. Pat. No. 6,023,929. Thus, the control system may determine that it is desirable to transition from operating one set of half the cylinders on the engine to the other half while remaining in cylinder fuel cut-out operation.
However, the inventors herein have recognized a disadvantage with such an approach. For example, looking at the example of an eight cylinder engine, and numbering the cylinders on the engine in firing order, the system may transition from firing 1, 3, 5, 7 to 2, 4, 6, 8. During this transition, two cylinders may fire in succession. If the torque produced by all the cylinders during the transition is substantially the same, the cycle-average torque produced during the transition may be higher than desired. This torque disturbance may also result in an engine speed disturbance if occurring during idle speed control conditions.
In one embodiment, it may be possible to reduce this disturbance by using a method for controlling an engine having a first group of cylinders and a second group of cylinders, the method comprising:
operating in a first mode with said first group combusting inducted air and injected fuel and said second group operating with inducted air and substantially no injected fuel;
transitioning to a second mode with said second group combusting inducted air and injected fuel and said first group operating with inducted air and substantially no injected fuel;
before said transition, reducing torque of a last to be combusted cylinder in said first group compared with a previously combusted cylinder in said first group; and
after said transition, reducing torque of a first to be combusted cylinder in said second group compared with a next combusted cylinder in said second group.